All-Metal Metamaterial-Based Sensor with Novel Geometry and Enhanced Sensing Capability at Terahertz Frequency

At a Glance

Metadata	Details
Publication Date	2025-01-16
Journal	Sensors
Authors	Sagnik Banerjee, Ishani Ghosh, C. Santini, Fabio Mangini, Rocco Citroni
Institutions	Sapienza University of Rome
Citations	8
Analysis	Full AI Review Included

Technical Analysis and Documentation: THz Metamaterial Sensor

Executive Summary

This technical documentation analyzes a novel all-metal metamaterial absorber (MMA) designed for high-sensitivity refractive index sensing in the Terahertz (THz) frequency range (5-8 THz).

Novel Geometry: The design utilizes four concentric, diamond-shaped gold resonators arranged in a unique step-pyramidal structure, with resonator height increasing from the outer (6 µm) to the inner (12 µm) rings.
Enhanced Performance: This geometry achieved six ultra-narrow absorption peaks, with a maximum absorptivity of 99.98% at 6.977 THz.
High Sensitivity: The sensor demonstrated an outstanding average sensitivity of 7.57 THz/RIU (Refractive Index Unit), significantly exceeding conventional triple-layered metamaterials.
High Quality Factor (Q): The structure exhibits very high Q-factors (up to 793.4), confirming its potential for ultra-narrowband filtering and high-resolution sensing applications (e.g., harmful gas detection, biosensing).
Robustness: The design is polarization-insensitive (up to 90° angle) and maintains stable absorption performance up to a 60° angle of incidence.
6CCVD Relevance: Replicating this high-fidelity, sub-100 µm unit cell requires advanced metalization and patterning capabilities, which are core strengths of 6CCVD’s MPCVD diamond platform.

Technical Specifications

The following hard data points were extracted from the research paper, detailing the performance and geometry of the proposed THz MMA sensor.

Parameter	Value	Unit	Context
Maximum Absorptivity	99.98	%	Achieved at 6.977 THz (Peak 3)
Number of Absorption Peaks	6	-	Hexa-narrowband performance
Operating Frequency Range	5 to 8	THz	Targeting the “Terahertz Gap”
Average Sensitivity	7.57	THz/RIU	High sensitivity for trace gas detection
Maximum Sensitivity	11.0357	THz/RIU	Achieved at 7.934 THz (Peak 6)
Maximum Quality Factor (Q)	793.4	-	Achieved at 7.934 THz
Unit Cell Periodicity (u)	86	µm	Optimized dimension for impedance matching
Ground Plate Thickness (t)	2	µm	Gold (Au) metal plate
Resonator Ring Thickness (a)	2	µm	Critical feature size for patterning
Resonator Heights (b)	6, 8, 10, 12	µm	Step-pyramidal geometry (outer to inner)
Polarization Insensitivity	Up to 90	°	Stable absorption response
Incidence Angle Stability	Up to 60	°	Stable absorption response
Gold Conductivity	4.56 x 10⁷	Sm^-1	Material property used in simulation

Key Methodologies

The experiment relied on precise material selection, novel geometry, and advanced electromagnetic simulation.

Material Selection: The structure is composed entirely of gold (Au) resonators and a gold ground plate, leveraging gold’s high conductivity (4.56 x 10⁷ Sm^-1) and plasmonic behavior in the THz range.
Novel Geometry Implementation: Four concentric square-ring resonators were designed with a fixed ring thickness (a = 2 µm) but varying heights (b). The height increases in 2 µm steps from the outermost ring (6 µm) to the innermost ring (12 µm), creating a distinctive step-pyramidal profile.
Unit Cell Optimization: Parametric sweeps were performed on key geometrical parameters (unit cell periodicity u, ground plate thickness t, and outermost ring height b) to achieve optimal impedance matching with free space (377 Ω) and maximize absorptivity.
Absorption Mechanism: Perfect absorption (A_w = 1 - R_w) was achieved by ensuring zero transmission (due to the thick metal ground plane) and minimizing reflection through impedance matching at the resonance frequencies.
Simulation Environment: Numerical simulations were conducted using CST Microwave Studio, employing the finite integration technique and tetrahedral meshing, with periodic boundary conditions imposed on the unit cell.
Sensing Quantification: Sensitivity (S) was calculated by measuring the shift in resonant frequency (Δf) relative to changes in the surrounding medium’s refractive index (Δn), quantified by the slope of the linear fit (S = Δf/Δn).

6CCVD Solutions & Capabilities

The successful fabrication and scaling of this high-performance THz sensor rely heavily on precision material deposition and patterning, areas where 6CCVD offers distinct advantages, particularly when transitioning from simulation to robust, scalable hardware.

Applicable Materials

While the paper utilized an all-metal gold structure, 6CCVD’s expertise in diamond materials provides critical advantages for next-generation THz devices, especially in applications requiring high thermal stability or integration into complex systems.

Optical Grade SCD/PCD: For applications requiring a robust, low-loss substrate beneath the metallic structure (e.g., for mechanical support or thermal management in energy harvesting), 6CCVD offers Optical Grade Single Crystal Diamond (SCD) or Polycrystalline Diamond (PCD) substrates. Diamond exhibits extremely low loss tangent and high thermal conductivity, making it superior to conventional dielectrics in the THz range.
BDD (Boron-Doped Diamond): For designs requiring a tunable or conductive ground plane, 6CCVD can supply Boron-Doped Diamond (BDD) films, offering precise control over conductivity and doping levels.

Customization Potential

The paper’s design requires precise control over micron-scale features and multi-layer metal deposition, capabilities central to 6CCVD’s advanced fabrication services.

Research Requirement	6CCVD Customization Capability	Impact on Research Extension
High-Fidelity Gold Resonators (2 µm features)	Precision Metalization: 6CCVD provides in-house deposition of high-purity Gold (Au), along with adhesion layers (e.g., Ti/Pt/Au stacks). We guarantee feature resolution necessary to replicate the 2 µm ring thickness and the complex step-pyramidal geometry.	Ensures accurate replication of the resonant structure, maintaining the high Q-factors (up to 793.4) and narrow FWHM required for high-resolution sensing.
Large-Area Array Scaling (86 µm periodicity)	Custom Dimensions: We supply PCD wafers up to 125mm in diameter. This capability allows researchers to scale the 86 µm unit cell into massive arrays, essential for integrated sensing and communications (ISAC) or m-MIMO applications in 6G networks.	Enables industrial scale-up and high-throughput manufacturing of THz sensor arrays.
Surface Quality for Sensing	Ultra-Smooth Polishing: Our SCD wafers achieve surface roughness Ra < 1nm, and inch-size PCD achieves Ra < 5nm. This ultra-smooth surface is critical for minimizing scattering losses and maximizing the interaction of the analyte with the THz field.	Directly enhances the measured sensitivity (7.57 THz/RIU average) by optimizing the sensing interface.
Complex Geometry Fabrication	Advanced Etching and Laser Cutting: 6CCVD offers custom laser cutting and etching services to define the precise 86 µm unit cell periodicity and the overall plate dimensions required for experimental setup.	Provides rapid prototyping and precise dimensional control for complex THz structures.

Engineering Support

6CCVD’s in-house team of PhD material scientists and engineers specializes in optimizing diamond properties for extreme applications. We offer consultation services to researchers looking to extend this work:

Material Optimization: Assistance in selecting the optimal diamond grade (SCD vs. PCD) and thickness (0.1 µm to 500 µm) to integrate with THz metamaterial designs, balancing optical transparency, thermal management, and cost.
THz Integration: Support in designing robust metalization schemes (e.g., Ti/Pt/Au) that adhere reliably to diamond surfaces under varying thermal and mechanical loads, crucial for long-term sensor stability.
Application Extension: Consultation for similar projects targeting THz spectroscopy, biomolecular fingerprint detection, and high-frequency communications hardware.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

This research proposes an all-metal metamaterial-based absorber with a novel geometry capable of refractive index sensing in the terahertz (THz) range. The structure consists of four concentric diamond-shaped gold resonators on the top of a gold metal plate; the resonators increase in height by 2 µm moving from the outer to the inner resonators, making the design distinctive. This novel configuration has played a very significant role in achieving multiple ultra-narrow resonant absorption peaks that produce very high sensitivity when employed as a refractive index sensor. Numerical simulations demonstrate that it can achieve six significant ultra-narrow absorption peaks within the frequency range of 5 to 8 THz. The sensor has a maximum absorptivity of 99.98% at 6.97 THz. The proposed absorber also produces very high-quality factors at each resonance. The average sensitivity is 7.57/Refractive Index Unit (THz/RIU), which is significantly high when compared to the current state of the art. This high sensitivity is instrumental in detecting smaller traces of samples that have very correlated refractive indices, like several harmful gases. Hence, the proposed metamaterial-based sensor can be used as a potential gas detector at terahertz frequency. Furthermore, the structure proves to be polarization-insensitive and produces a stable absorption response when the angle of incidence is increased up to 60°. At terahertz wavelength, the proposed design can be used for any value of the aforementioned angles, targeting THz spectroscopy-based biomolecular fingerprint detection and energy harvesting applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s25020507

References

2022 - A Biomedical Sensor for Detection of Cancer Cells Based on Terahertz Metamaterial Absorber [Crossref]
2005 - Properties of left-handed metamaterials: Transmission, backward phase, negative refraction, and focusing [Crossref]
2021 - A perfect selective metamaterial absorber for high-temperature solar energy harvesting [Crossref]
2020 - Terahertz absorption modulator with largely tunable bandwidth and intensity [Crossref]
2021 - Multi-band terahertz resonant absorption based on an all-dielectric grating metasurface for chlorpyrifos sensing [Crossref]
2017 - Solid analyte and aqueous solutions sensing based on a flexible terahertz dual-band metamaterial absorber [Crossref]
2018 - Design of a metasurface-based dual-band Terahertz perfect absorber with very high Q-factors for sensing applications [Crossref]
2021 - Wave-thermal effect of a temperature-tunable terahertz absorber [Crossref]
2024 - Ultra-broadband and wide-angle plasmonic absorber based on all-dielectric gallium arsenide pyramid nanostructure for full solar radiation spectrum range [Crossref]